Zircon megacrysts from basalts of the Venetian Volcanic Province (NE Italy) : U-Pb ages, oxygen isotopes and REE data

Mafic alkaline lava from the Venetian Volcanic Province (NE Italy) contain orange-brown zircon megacrysts up to 15 mm long, which are subhedral to subrounded and show equant morphology, with width-to-length ratios of 1:2-1:2.5. U-Pb ages of zircon (51.1±1.5 to 30.5±0.51 Ma) fit the stratigraphic age of the host alkaline mafic lava (Middle Eocene and Oligocene) and the oxygen isotope composition (delta 18 O = 5.31-5.51 ‰) is similar to that of zircon forming in the upper mantle. Cathodoluminescence images and crystal chemical features, e.g., depletion of incompatible elements such as REE, Y, U and Th at constant Hf content, indicate that core-to-rim zircon zoning is not consistent with evolution of the melt by fractional crystallization. On the contrary, all the above features agree in indicating that the subhedral morphology is related to the genesis of the host alkaline mafic magma. The widespread presence of subhedral zircon in mafic and ultramafic rocks indicates that metasomatism of early mantle magmas may be invoked as a common process in the generation of basic alkaline magma.